In recent years, digital documentation has advanced along with the prevalence of scanners. In order to store a digital document in a full-color bitmap format, for example, an A4-size document amounts to about 24 Mbytes at 300 dpi, and a huge memory size is required. Such large-size data is not suitable for sending it as an attachment file of an e-mail message. To solve this problem, it is a common practice to compress a full-color image, and JPEG is known as a compression method for such purpose. JPEG is very effective to compress a natural image such as a photo or the like, and can assure high image quality. However, when a high-frequency portion such as a text portion is compressed by JPEG, image deterioration called mosquito noise is generated, and the compression ratio is low. Hence, the image is broken up into some areas to generate JPEG-compressed data of a background portion except for a text area, and MMR-compressed data of a text area portion with color information. Upon decompression, the individual data are decompressed and combined to express an original image.
However, a compression ratio that can be realized by the aforementioned method while maintaining high image quality is not high enough. Also, information of characters emphasized in red in black character text is lost. That is, when an image containing a text portion using two or more colors is compressed by the above compression method and the compressed image is expanded, the number of colors of the text portion contained in the expanded image is reduced to one.